The invention relates to the field of signal coding in a hybrid channel.
In some source coding scenarios, there exist observations of signals at the decoder that are correlated with the source which may be used jointly with a digital representation to reconstruct the source. For example, in the case of in-band on-channel (IBOC) digital audio broadcast (DAB), an existing noisy analog communications infrastructure may be augmented by a low-bandwidth digital side channel for improved fidelity. As another example, in a two-sensor scenario, one sensor may observe a distorted full-bandwidth form of the source signal, while the other observes the source undistorted but can only record or transmit a low-bandwidth representation of the signal. A final example is a source coding scheme that devotes a fraction of available bandwidth to the analog source and the rest of the bandwidth to a digital representation. This scheme is applicable in a wireless communications environment, where analog transmission has the advantage of a gentle xe2x80x9croll-offxe2x80x9d of fidelity with SNR.
The basic model representing such systems, which is referred to as the xe2x80x9chybrid channelxe2x80x9d, is illustrated in FIG. 1. The hybrid channel consists of a noisy analog channel 102, through which a signal source 104 is sent unprocessed, and a secondary rate-constrained digital channel 106. The source is processed by an encoder 108 prior to transmission through the digital channel. A receiver 110 estimates the source from both the analog and digital data. It is assumed that no processing is performed prior to transmission over the analog channel 102. Any form of modulation, such as amplitude or frequency modulation, is assumed to be part of the analog channel model.
The invention provides an apparatus and method for subband signal coding, using algorithms of comparable complexity to conventional coders, that exploits a noisy analog signal at the decoder. It is assumed that the analog signal is the output of a channel through which the source is sent uncoded. By using the analog signal at the receiver, the required digital bit rate should be able to be reduced while offering comparable fidelity to conventional coding systems that ignore the analog signal. In the DAB scenario, broadcasters can use the bits saved on audio source coding either for improved error-correction or transmission of non-audio data.
The term xe2x80x9csystematicxe2x80x9d has been used to describe source coding with analog information at the receiver as an extension of a concept from error-correcting channel codes. A systematic error-correcting code is one whose codewords are the concatenation of the uncoded information source string and a string of parity-check bits. Similarly, in the systematic hybrid source coding scenario, there is an uncoded analog transmission and a source-coded digital transmission.
In accordance with the invention, concepts from conventional subband coding, e.g., subband decomposition, quantization, bit allocation, and lossless bitstream coding, are tailored to exploit the analog signal at the receiver such that frequency-weighted mean-squared error (MSE) is minimized. Because subband coefficients are coded, all results pertaining to perceptual masking are easily applied to this method of coding. In addition, the techniques of the invention require very little additional overhead as far as source side information. Although the results are applicable to coding of all signals, the application of these digital coding techniques to the perceptual coding of audio as a solution to the DAB problem is emphasized. Using a 30 dB analog signal corrupted by additive white Gaussian noise at the decoder, bit rates as low as 10 to 20 kbits/sec are attainable for transparent coding of mono audio sampled at 44.1 kHz.
The invention is directed to a signal coding solution for a hybrid channel that is the composition of two channels: a noisy analog channel through which a signal source is sent unprocessed and a secondary rate-constrained digital channel. The source is processed prior to transmission through the digital channel. Signal coding solutions for this hybrid channel are clearly applicable to the in-band on-channel (IBOC) digital audio broadcast (DAB) problem. A perceptually-based subband audio coder is provided, with complexity comparable to conventional coders, that exploits a signal at the receiver of the form y[n]=g[n]*x[n]+u[n], where x[n], g[n], and u[n] denote respectively the source, the impulse response of convolutional distortion, and additive Gaussian noise.
Accordingly, in one exemplary embodiment the invention provides a systematic hybrid analog/digital encoder, and corresponding method of encoding, which processes data including an analog source signal which is transmitted on an analog channel and a digital source signal whose digital encoding is transmitted on a digital channel, the digital source signal being a discrete-time sampled signal of the analog source signal. The encoder comprises an analysis filter bank which performs a subband decomposition of the digital source signal to generate a plurality of subband source signals; a quantizer which processes the plurality of subband source signals, based on characteristics associated with the analog channel and the characteristics associated with the digital source signal, to generate a plurality of quantizer output levels represented by a sequence of bits; a lossless bitstream coder which processes the sequence of bits as a function of the analog channel characteristics and the digital source signal characteristics, to generate an output coded bitstream; and a bitstream formatter which integrates the coded bitstream with supplementary data associated with the subband source signals.
In another exemplary embodiment, the invention provides a systematic hybrid analog/digital decoder which processes data received from analog and digital channels, the analog channel having an analog output signal related to an analog source signal, and the digital channel having a formatted bitstream derived from a digital source signal. The decoder comprises a bitstream interpreter which reads the bitstream and determines a coded bitstream and supplementary data associated with a plurality of subband source signals derived from the digital source signal; an analog estimiator that processes the analog output signal based on characteristics associated with the analog channel and characteristics associated with the digital source signal, to generate a plurality of subband signal estimates; a bitstream decoder which decodes the coded bitstream based on the analog output signal, characteristics associated with the analog channel, and characteristics associated with the digital source signal, to generate a plurality of quantizer output levels; a subband signal generator which generates a plurality of reconstructed subband signals based on the subband signal estimates and the quantizer output levels; and a reconstructed source generator which generates a reconstructed digital source signal by processing the reconstructed subband signals with a synthesis filter bank.